The present invention relates generally to hydrogen-permeable membranes, and more particularly to a hydrogen-permeable membrane with increased hydrogen permeability and a method for producing the same.
In particular, planar palladium-alloy membranes have been disclosed for purifying hydrogen gas streams, such as hydrogen gas streams produced by steam reformers, autothermal reformers, partial oxidation reactors, pyrolysis reactors and other fuel processors, including fuel processors configured to supply purified hydrogen to fuel cells or other processes requiring high-purity hydrogen.
To be economical, palladium and palladium-alloy membranes must be thin. For example, planar membranes are typically approximately 0.001 inches thick. However, these extremely thin membranes are subject to wrinkling during assembly into a form for use in a fuel processing system, such as assembly into a membrane module containing a plurality of membranes. A membrane that has one or more wrinkles is subject to premature failure due to stress fractures forming at the wrinkle. When such a failure occurs, impurities that otherwise would be unable to pass through the membrane can now pass through the membrane, thereby reducing the purity of the product hydrogen stream and potentially damaging the fuel cell stack or other device with which the purified stream is used.
Purified hydrogen is used in the manufacture of many products including metals, edible fats and oils, and semiconductors and microelectronics. Purified hydrogen is also an important fuel source for many energy conversion devices, such as fuel-cell systems, and especially proton-exchange-membrane fuel-cell (PEMFC) systems.
Hydrogen gas streams may be produced by fuel processors that produce hydrogen gas by chemically reacting one or more feed streams. These fuel processors often require that the initial hydrogen stream be purified before the stream is suitable for use in a particular application, such as a feed stream to a fuel cell.
An example of a suitable fuel processor is a steam reformer, which produces hydrogen gas by reacting a hydrocarbon or alcohol with water. Other examples of suitable fuel processors produce hydrogen gas by autothermal reforming, partial oxidation of a hydrocarbon or alcohol vapor, by a combination of partial oxidation and steam reforming a hydrocarbon or an alcohol vapor, by pyrolysis of a hydrocarbon or alcohol vapor, and by electrolysis of water. Examples of suitable fuel processors and fuel cell systems incorporating the same are disclosed in U.S. Pat. Nos. 5,861,137 and 5,997,594, and U.S. patent application Ser. No. 09/190,917, now U.S. Pat. No. 6,376,113, which was filed on Nov. 12, 1998 and is entitled xe2x80x9cFuel Processing System,xe2x80x9d and the disclosures of which are hereby incorporated by reference.
The invention includes a hydrogen-permeable metal membrane with increased hydrogen flux compared to conventional hydrogen-permeable metal membranes. Without sacrificing selectivity or purity, the membrane enables a greater hydrogen throughput. A method for preparing the membrane includes at least one etching step in which a volume of etchant is used to selectively remove material from the membrane""s surface. Methods for detecting and repairing holes or other defects in the membrane are also disclosed. The invention also includes a membrane module adapted for use in purifying hydrogen streams, such as those produced by fuel processors.